1. Field of the Invention
In certain medical and security related environments it is desirable to create a signal when certain persons exit a designated environment. For example, in a nursing home environment it is desirable that a signal be created if a selected patient or resident exits the nursing home. In a similar manner, it is desirable that a signal be created if a person exits their bed under circumstances where such exit would be deemed an unsafe or undesirable activity.
Most of the existing detection systems utilize radio frequency (RF) techniques wherein the object (person to be detected) is fitted with a small RF transmitter and the control unit (or detection device) is installed adjacent to a monitored passageway such as a building exit. These systems detect the monitored person by reception and recognition of the unique frequency associated with the person, to the exclusion of all other persons not emitting this pattern. The complexity of these systems varys from the non-sophisticated to the very complex pattern (or signal analysis) type. These systems may be very expensive, depending upon the degree of complexity, which is normally related to the desire of reducing the occurrence of false alarms. Many of these systems utilize batteries in an RF transmitter which is fitted to the patient, either on the wrist or at another location, which batteries must be periodically replaced. In some cases, the entire transmitter assembly must be replaced, thereby leading to fairly expensive recurring costs in the use of the system and many systems utilize detection elements such as magnets in both shoes of the person monitored.
Many such systems are currently available on the market and typical of these is the "Code Alert" wanderer monitoring system developed by R. F. Technologies, Inc., of Milwaukee, Wis. A similar system known as "Wander Guard" selective alert system includes a signalling device worn by the wanderer and a receiver mounted in the door or recessed in the wall of a structure. If a wanderer walks through the monitored area, a distinctive beeper alerts nursing staff or security to the movement of the patient or resident. The "Wander Guard" system is developed by Senior Technologies, Inc., of Lincoln, Nebr. A similar system is "Smart Lock" which is designed to lock an exit only when approached by a monitored resident. When the resident retreats from the door, the "Smart Lock" control quietly returns the door to its unlocked state. The "Smart Lock" system is developed by McKerly Company of Concord, N.H.
Many such detectors are the subject of United States patents, typical of which is the "Passageway Selective Detector Mechanism and System" detailed in U.S. Pat. No. 4,555,696, dated Nov. 26, 1985, to Donald G. Brown. The passageway selector detector mechanism and system includes a mat which is positioned within a passageway such as a doorway, gate or the like. Embedded within the mat are magnetically operable electric switches which are electrically connected to an alarm or a signal member and to a source of electrical energy. Thus, when any of the switches is operated, the alarm or signal members are energized. The mechanism and system includes magnetic elements located in the shoes of selected personnel for operation of at least one of the electric switches as the selective personnel travels over the mat. The magnetic element may include an inner sole within the shoes of the selected personnel to actuate an electric switch, which magnetic element actuates the alarm or signal member as the selective personnel steps upon the mat. The magnetic element may also include a magnetic member carried by a wheelchair in which the selective personnel travels and which actuates an electric switch, which energizes an alarm or signal member as the selective personnel travels over the mat in the wheelchair. U.S. Pat. No. 4,945,340, dated Jul. 31, 1990, to Henry L. Brille, details a "Tamper-Resistant Magnetic Security System". The security system is adapted for use in a physical security monitoring environment and includes a switch unit having a common conductor a guard conductor and at least three switches. Each of the switches incorporates a deactivated condition and an activated condition and each is adapted to be placed in its activated condition in response to a magnetic field of predetermined magnetic flux. The switch unit also includes a logic circuit electrically interconnecting the switches and the common and guard conductors, the logic circuit completing a series circuit between the common conductor and the guard conductor whenever at least two predetermined, but not all, switches are in magnetically actuated condition. As associated actuated unit includes at least two permanent magnets and provides discreet magnetic fields of predetermined flux density and positioned to individually activate the two or more predetermined switches and complete the series circuit between the common conductor and the guard conductor when the actuator unit is located in predetermined juxtaposition with respect to the sensor unit. A "Frequency Divider With Single Resonant Circuit and Use Thereof As a Transponder in a Presence Detection System" is detailed in U.S. Pat. No. 4,654,641, dated Mar. 31, 1987, to L. G. Ferguson, et al. The device includes a portable frequency divider which includes a single resonant circuit including a nonlinear conductor having a core made of amorphous magnetic material and a capacitance connected in series with the conductor, to define a resonant circuit that detects electromagnetic radiation at a first predetermined frequency and responds to the detection by transmitting electromagnetic radiation at a second frequency which is a subharmonic of the first frequency. U.S. Pat. No. 4,663,612, dated May 5, 1987, to E. Majia, et al, details a "Pattern-Comparing Security Tag Detection System". The security apparatus is designed to detect a security tag which is hidden in an article to be detected. The digitized wave shape of a tag signature signal is preferably obtained by distortion of an oscillating magnetic field (whereby the distortion signal being detected constitutes a signature signal) and is compared with a stored digital representative of a signature signal and an alarm enabling signal is generated in the event of correlation in a predetermined number of times in a predetermined time period. The storage signal is modified by the specific factors and characteristics of the apparatus, as well as environmental factors. U.S. Pat. No. 4,882,569, dated Nov. 21, 1989, to S. Dey, covers a "De-Activatable Frequency-Dividing-Transponder Tag". Detailed is a presence-detection-system tag in which a frequency-dividing transponder may be decisively deactivated notwithstanding the intensity of the ambient magnetic field. The tag includes a frequency-dividing transponder, including an active strip of magnetic material which, when magnetically biased to be within a predetermined magnetic field intensity range, responds to excitation by electromagnetic radiation of a first predetermined frequency by radiating electromagnetic radiation of a second predetermined frequency that is, a frequency-divided quotient of the first predetermined frequency. A first biased strip of magnetic material is disposed relative to the active strip of magnetic material for biasing the active strip of magnetic material within the predetermined magnetic field intensity range, only when the first biased strip is magnetized. Further included is a second biased strip of magnetic material disposed relative to the active strip of magnetic material for further biasing the active strip to be outside of the predetermined magnetic field intensity range and thereby preventing the active strip of magnetic material from radiating electromagnetic radiation of the second predetermined frequency in response to excitation by electromagnetic radiation of the first predetermined frequency when the first and second bias strips of magnetic material are both magnetized. A coded tag includes such two active strips having different magnetomechanical resonant frequencies and either tag may be detected in a presence detection system that includes means for transmitting an electromagnetic radiation signal of a first predetermined frequency into a surveillance zone and means for detecting electromagnetic radiation of the second predetermined frequency within the surveillance zone. The system further includes a magnetizer for magnetizing the second bias strip to deactivate the frequency-dividing transponder of the tag.
One of the problems which exists with existing patient monitoring and control systems is the problem of properly and efficiently fitting the patient or the patients' shoes with a desirable transmitter or other detection device which cannot be removed. For example, in a nursing home or hospital environment, certain residents or patients may not have a cognitive awareness of or concern for their attire. Most of the people in this category suffer from some type of dementia such as Alzheimer's, or in some cases, brain injury. Consequently, the residents or patients cannot be relied upon to wear the required devices or their shoes when they attempt an unauthorized exit from their room in the hospital, nursing home or other facility.
The "Hall Effect" integrated circuit (IC) offers promise for use in a detection system, since it is capable of sensing a magnetic field. Specifically, this device detects motion, position or change in the field strength of an electromagnet, permanent magnet or a ferromagnetic material with an applied magnetic bias. However, manufacturers specifications normally limit the useful sensitivity or range of "Hall Effect" IC switches and sensors to an extremely small distance of about 1/2 of an inch. With increased complexity and the use of strong magnetic assemblies, this sensitivity can be extended to about 1 inch. In contrast, the selective exit control system of this invention is capable of detecting human movement by actually detecting the movement of magnetic material in a manner which yields a sensitivity of up to ten times the normal sensitivity of the conventional "Hall Effect" IC switch. This increased sensitivity is absolutely necessary to the efficacy of the invention, since the passage of a person's foot over a floor-mounted object such as a threshold or the like, is variable in terms of linear clearance and averages from about two to three inches. It should be noted that increases in "gain" and increases in "sensitivity" from an electronic standpoint are not synonomous. Increases in "gain" are relatively easy to obtain, while even minor increases in sensitivity are extremely difficult to achieve. The sensitivity of solid state "Hall Effect" IC switches is generally controlled by the manufacturer. This tends to limit use of these switches to standard "textbook" applications. When these devices "see" a state of minimum magnetic field strength they will switch in a predictable manner.
Accordingly, the first important component of the selective exit control system of this invention is use of a "Hall Effect" IC sensor which has a continuously variable output related to the magnetic field strength that it "sees". In this regard, it performs an analog rather than a digital switch function. However, the "Hall Effect" IC sensor can "see" a very small change in magnetic field strength, but the resulting signal is of such small magnitude as to be virtually unusable.
The second important component of the invention is based on the understanding that the "unusable" signal generated by the "Hall Effect" IC sensor can be essentially doubled, that is, increased in sensitivity, by using two "Hall Effect" IC sensors instead of one, and configuring them in an electronic "push-pull" arrangement, such that one output is positive as the other output is negative for the same magnetic stimulus. Each of these "Hall Effect" IC outputs is connected to one of two "stage one" operational amplifier (buffer-amplifier) inputs. In this manner, the "stage 1" buffer/amplifier resistors can be conventionally selected to provide a selected "gain".
The third important component of the invention is the use of a pair of mutually reinforcing, permanent magnet assemblies which include exceptionally strong "rare earth" magnets that are both oriented to simultaneously increase and broaden the magnetic field strength in a preferred direction, in this case, downwardly. These magnets assist in converting the low, "unusable" signal into levels which can be further processed in a useable manner according to the invention.
The fourth important component of the invention includes implementation of a comparator circuit. The normal use of comparators requires two separate inputs, one of which is a fixed reference and the other a variable input, such that if the variable input goes slightly above the fixed reference, the comparator will change its output state. This standard design was unacceptable for use in the selective exit control system of this invention because the absolute difference between the two input voltages was very small. Accordingly, any drift or slight deviation in the input over time would cause the results to lack stability and repeatability. The solution was to design the circuit such that the fixed reference is not a fixed voltage level, but is derived from the variable input.
The fifth important aspect of the invention is the selection of component values in the electronic arrangement such that the circuit allows "finger-printing" or signature analysis of the wave-shape characteristics insofar as they are representative of expected or actual human movement. Since the two comparator inputs are derived from the same point, signals which are too fast or too slow are purposely ignored by the detection circuit. This discrimination against signals which appear not to be representative of human movement is provided by the unique arrangement and choice of component values in the invention.
Accordingly, it is an object of this invention to provide a non-mechanical, electronic and magnetic detection system which creates a signal when a selected person wearing a single, removable or non-removable, fitted cuff or anklet containing one or more magnetic elements, walks through a detection zone.
Another object of this invention is to provide an electronic system for detecting wandering patients or residents by utilizing a combination of strong "rare earth" magnets fitted on an anklet or cuff in non-removable fashion and effecting an increased field strength, such that solid state "Hall Effect" magnetic sensors, using special techniques which enhance detection sensitivity, may detect and create a signal over a distance of up to about 10 times that normally associated with magnetic sensing devices.
Still another object of this invention is to provide a non-mechanical selective exit control system which includes a non-battery operated magnetic anklet designed for mounting on the ankle of a patient or resident and fitted with a pair of spaced rare earth magnets disposed so as to increase the field strength of the magnets and further including a "Hall Effect" integrated circuit (IC) detection system which is enhanced electronically and fitted within a threshold strip to facilitate detection of the magnetic anklet of the patient or resident when he or she steps through the detection zone generated by the enhanced "Hall Effect" IC sensors.